In order to detect radiation, many different principles are used in various types of radiation detectors depending on the application field and the range. For example, there are several devices including a gas filled counter using the method of electrolytic dissociation of vapor molecules by radiation; a semiconductor detector using the method of generating electron-hole pair from semiconductor exposed to radiation; a scintillation counter using materials that emit light when exposed to radiation; a film badge using the reaction of films to radiation; and a thermoluminescence dosimeter (hereinafter, referred to as “TLD”) using the method of irradiating an insulator or a semiconductor so as to get thermostimulated and become luminescent.
Among the various radiation detectors, the TLD is widely used as a radiation detector to measure the personal exposed dose. In the thermoluminescence process, when an insulator or a semiconductor is irradiated by high-energy radiation from the exterior, electrons in the valence band are excited to the conduction band. Some of the electrons go back to the valence band in a very short period of time, however, some other electrons stay in the trapping level of energy in the forbidden band due to impurity and defect of the solid. If there is no thermal disturbance, the electrons in the trapping level accumulate radiation doses in a stable energy state for a long period of time. Using this property, the accumulated radiation dose can be measured, thus enabling management of the radiation dose accumulated inside the body of a worker exposed to radiation in an environment of high radiation, and the monitoring of an environment dose as well.
However, if the electrons take thermal energy from the exterior, they no longer remain in the trapping level, and they go up to the conduction band. At that stage, the electrons react with holes in the recombining level in the forbidden band, and generate the corresponding energy of light, which can be utilized as dosimeter since the amount of luminescence of the light is proportionate to the radiation dose in a certain region. Using the light emitted at this stage, a personal exposed dose can be measured or can be used medically for diagnosis and treatment of a patient.
In order to prepare a high-quality TLD, the thermoluminescent elements comprising of thermoluminescent materials should have excellent sensitivity to sufficiently low radiation, and have the most preferred glow curve structure.
In accordance with ICRP 60 [ICRP, 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, Pergamon Press, Oxford, New York, 1990], it is necessary for the thermoluminescent materials to be highly sensitive to a radiation dose as low as reasonably achievable (ALARA).
The glow curve represents the relative thermoluminescent intensity corresponding to the heat stimulus temperature. The information of the radiation dose is generally extracted from the integral area under the glow curve. That is, since the area of the glow curve means the amount of luminescence and the amount is proportionate to the radiation dose, the area of the glow curve appears as the accumulated radiation through the radiation evaluation algorithm including various compensating values.
Thermoluminescence is a phenomenon wherein some of irradiated electrons stabilize in the trapping level, and then, when heated, emit light. Electrons corresponding to the luminescence peak that are generated in the low temperature area can be easily excited even in room temperature. This result means the possibility that the electrons might get excited in room temperature increases, as the temperature of the luminescence peak gets lower.
Even though one of the important advantages of a TLD is the ability to evaluate the accumulated radiation dose, a TLD having a luminescent peak in the low temperature area does not satisfy the requirement and loses reliance as a radiation detector. Because the TLD cannot maintain complete information on the dose of radiation, but loses some of the information when it is irradiated and stays in room temperature for a certain period of time.
Therefore, the preferred glow curve characteristics, i.e., the location of the main glow peak and its width, should appear in the high temperature area rather than in the low temperature area, and have a simple and single main peak, and, at the same time, no peak should be found in the low temperature area if possible.
Recently, there are many researches to develop thermoluminescent materials that have high sensitivity to sufficiently low radiation and show a glow curve with a simple and single main peak.
The glow curve structure greatly depends on the state of the trapping level in the forbidden band formed by the types or the concentrations of the added impurities. In order to obtain the most preferable thermoluminescent materials, the optimum types or their concentrations of impurities should be found.
A well-established and widely employed dosimetry technique is thermoluminescence dosimetry (TLD) using LiF as the host material. Presently, America, China, Poland, and France take the initiative to research into LiF elements. Depending on the purpose, they might be classified to a powder type or a pellet type. Examples of such LiF elements for a TLD are GR-200A comprised of LiF:Mg,Cu,P composition which is commercialized in China and MCP-N comprised of LiF:Mg,Cu,P composition which is commercialized in Poland. Recently, the compositions of these commercially available TLDs, LiF:Mg,Cu,P, are spotlighted.
The present inventors had already prepared a powder type LiF element for a TLD, showing high sensitivity and a simple and single main peak, which includes 0˜1.0 mole % of Mg source; 0˜1.0 mole % of Cu source; 0˜2.4 mole % of Na source; and 0˜2.4 mole % of Si source as dopants in LiF.
However, when the powder type LiF element having high sensitivity goes through a solidifying process, in which the powder type is changed into the pellet type by sintering, it cannot maintain its sensitivity. Therefore, most of thermoluminescent elements should be produced by a continuous process, from the starting elements to the final product of a pellet type.
The present inventors have prepared a pellet-type LiF element for a TLD which includes 0.35˜0.12% by mole of Mg source; 0.08˜0.001% by mole of Cu source; 1.3˜0.5% by mole of Na source; and 1.3˜0.5% by mole of Si source as dopants. The pellet-type LiF element according to the present invention shows high sensitivity to sufficiently low radiation and the most preferable glow curve.